The A4E tracker pulls several times within an hour the order book of a structure. For the income estimation new orders that haven't been seen in the previous pull are taken and a 0.1% broker fee of their ISK value (= price * quantity) recorded. NPC stations are calculated with 3% fee.

This value is inaccurate since a) orders may be filled very quickly that the tracker doesn't see them; b) the broker fee is not 0.1% and c) there are exceptions of the broker fee for individuals.